As cool as it might be to visit a zoo filled with woolly mammoths, saber-toothed tigers, and giant tortoises, the best reasons for bringing back extinct animals have more to do with ecology than tourism. “If this is always going to be a zoo animal, then stop,” says ecologist Ben Novak, the lead researcher on the passenger pigeon project at Revive & Restore—a foundation devoted to genetically rescuing endangered and extinct species in San Francisco, California. “The goals have to be about ecological restoration and function.”

Every animal in an ecosystem has a function: Bats eat insects, fish clean algae from coral, grazers spread nutrient-rich dung across habitats. Some functions are redundant —shared among multiple different animals—but others are fulfilled by just one or two species.

Both the passenger pigeon and woolly mammoth were functionally unique species, and when they went extinct, their habitats changed dramatically. Harvard University’s George Church, the lead researcher working to de-extinct the mammoth, says that bringing back the giants could help convert the Arctic tundra back to grasslands that existed during the last ice age. He points to research that shows that mammoths and other large herbivores trampling across the ancient Arctic ecosystems helped maintain the grasslands by knocking down trees and spreading grass seeds in the dung. When the large herbivores disappeared, the ecosystem transitioned to today’s mossy tundra and taiga that is beginning to melt and release carbon dioxide into the atmosphere. Reviving the mammoth, Church says, could help slow climate change by shifting the landscape back toward the grasslands. “There’s twice as much carbon at risk in the tundra than in all the forests of the world put together.”